1. Field of the Invention
The present invention relates generally to marine propulsion systems with two propellers on two propeller shafts that rotate about a common axis in which the two propellers rotate in opposite directions and, more specifically, to a marine propulsion system that rotates one of the two propellers at a different rotational speed than the other.
2. Description of the Prior Art
Many different types of marine propulsion systems are well known to those skilled in the art. A certain type of marine propulsion system mounts two counter-rotating propellers inline to rotate about a common axis of rotation.
Marine propulsion systems of this kind can be used in stern drive applications, where the engine is enclosed within the hull of a boat and the propellers are mounted on a unit attached to the transom and driven by a driveshaft extending through the transom. Alternatively, an outboard motor can be provided with dual counter-rotating propellers. The present invention is equally applicable to either a stern drive system or an outboard motor system.
U.S. Pat. 5,352,141, which issued to Shields et al on Oct. 4, 1994, discloses a marine drive with dual propeller exhaust and lubrication system. The drive has a bearing and seal housing which is called a spool and which is positioned in the lower horizontal bore and supporting a dual propeller shaft assembly. An exhaust passage includes a passage in the drive housing communicating with the horizontal bore at the spool, and a spool exhaust passage passing exhaust rearwardly through the spool to the propeller through-hub exhaust passages, providing through-hub exhaust through dual propellers. An inner oil passage in the spool communicates with the horizontal bore forewardly of the exhaust passage and lubricates the dual propeller shaft assembly.
U.S. Pat. No. 5,083,989, which issued to Yates et al on Jan. 28, 1992, describes a drive transmission which is particularly suitable for ship propulsion. The transmission comprises nested, contra-rotating propeller shafts which, in one example, are driven by a single input shaft and which transmits contra-rotating drives to the shaft via respective planet carriers. Particular advantages are the equal divisions of torque between the shafts and the enabling of location of thrust bearings in positions which are easy to access.
U.S. Pat. No. 5,366,398, which issued to Meisenburg et al on Nov. 22, 1994, discloses a marine dual propeller lower bore drive assembly. The marine drive has two counter-rotating propellers. Inner and outer concentric counter-rotating propeller shafts are supported by a spool in the lower horizontal bore. Passages are provided in the housing for communicating lubrication and/or exhaust with the horizontal bore. A locking structure holds the driven gears and bearings in place in the lower horizontal bore.
U.S. Pat. No. 5,601,464, which issued to Ogino et al on Feb. 11, 1997, describes a transmission system for a counter-rotational propulsion device. The transmission system is easily incorporated into an existing outboard drive of a watercraft in order to convert the outboard drive from a single propeller drive to a counter-rotational dual propeller system. The transmission system involves a first transmission which selectively couples an inner propulsion shaft with an existing driveshaft of the outboard drive. The inner propulsion shaft in turn drives a rear propeller. A second transmission of the transmission system is provided between the inner propulsion shaft and a outer propulsion shaft. The second transmission reverses the rotational drive direction input by the inner propulsion shaft so as to drive the outer propulsion shaft in an opposite rotational direction. The outer propulsion shaft drives a front propeller which spins in an opposite direction to that of the rear propeller, but exerts a thrust in the same direction as the rear propeller.
U.S. Pat. No. 5,230,644, which issued to Meisenburg et al on Jul. 27, 1993, discloses a counter-rotating surfacing marine drive. The drive has two counter-rotating surface operating propellers. An upper adapter spool has a lower threaded outer portion mating with a threaded portion of the vertical bore of the drive housing and supporting the upper gear for rotation about the driveshaft and supporting the driveshaft for rotation within the adapter spool. A vertical bore structure enables assembly from above of the majority of the vertical train components into a one piece unitary integrally cast housing. The vertical distance between the adapter spool and the lower bearings supporting the vertical driveshaft is approximately equal to the propeller radius. The lower concentric counter-rotating propellers shafts are spaced from the upper input shaft by a distance along the driveshaft in the range of approximately 9 to 15 inches.
U.S. Pat. No. 4,963,108, which issued to Koda et al on Oct. 16, 1990, describes a marine counter-rotating propeller drive system. The system comprises a large gear driven by an engine, and a plurality of small gears disposed so as to respectively mesh with the large gear at a plurality of fixed positions along the circumference of the large gear. Planet gears are respectively mounted to the gear shafts of the small gears. A sun gear and an inner tooth gear are respectively meshed with the planet gears. A rear propeller is mounted to an inner shaft serving as a gear shaft of the sun gear. A front propeller is mounted to a tubular outer shaft serving as a gear shaft of the inner tooth gear. The system can eliminate, with a simple construction, the inconvenience caused by a differential planetary gear operation as known in the prior art and can derive a propeller efficiency to a maximum extent.
U.S. Pat. No. 5,249,995, which issued to Meisenburg et al on October 5, 1993, discloses a marine drive having two counter-rotating surfacing propellers and dual propeller shaft assemblies. The marine drive has two counter-rotating surface operating propellers. Inner and outer concentric counter-rotating propeller shafts are supported by a spool assembly locked and retained against rotation and against axial movement in the lower horizontal bore in the torpedo of the drive housing by axially spaced left and right hand threads. A thrust bearing assembly transfers thrust from the outer propeller shaft to the inner propeller shaft during rotation of the propeller shafts in an opposite direction and is located between the fore and aft driven gears. Propeller shaft sealing and bearing structure and propeller self centering mounting structures are also provided.
In certain applications of dual propeller systems, it is beneficial to make the counter-rotating propellers rotate at different speeds. Some of the advantages that result from having the two propellers rotate at different speeds relate to improvements in the acceleration, top speed, and performance capability of the drive system. In addition, the two propellers can be provided with different pitches for improved maneuverability during at low speed docking procedures when the two propellers are rotated at different speeds. There are various reasons for designing a marine propulsion system in this way, but known designs require a degree of complexity that tends to make the propulsion system too expensive or unreliable. It would therefore be significantly beneficial if a counter-rotating marine propulsion system could provide a way in which the two propellers can be rotated at different speeds without requiring the undue complexity previously known to those skilled in the art.